Method of margin alignment and plane-to-plane registration in a tandem color electrophotographic machine

ABSTRACT

A method of correcting registration in an electrophotographic machine includes the steps of adjusting one of a right margin and a left margin by setting a time delay, determining a longest line length of a calibration pattern, and adjusting an other of the right margin and the left margin to match the longest line length.

This is a Divisional of application Ser. No. 09/795,768 filed Feb. 28,2001 now U.S. Pat. No. 6,549,225.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tandem color electrophotographicmachine, and, more particularly, to margin alignment and plane-to-planeregistration in a tandem color electrophotographic machine.

2. Description of the Related Art

Tandem color printing enables color electrophotographic printing to beperformed by a printer at the same speed as black-and-white (mono color)electrophotographic printing. Such a tandem process utilizes fourseparate imaging devices to simultaneously create four separate latentimages on four different photoconductive surfaces. Thus, all four colorscan be imaged, developed, and transferred to the desired media at thesame speed as a single color could be printed. Single color printers allimplement techniques to align the top of the page, left margin, linelength, and page length. In a tandem configuration, an additionaldifficult problem arises in registering each color image planeaccurately relative to all the other color image planes. Manufacturershave been hesitant to produce tandem color laser printers because of thedifficulty in maintaining alignment and plane-to-plane registration dueto manufacturing tolerances.

What is needed in the art is a method of aligning the registration ofall color planes in both the process direction (media direction oftravel) and the scan direction (cross process direction).

SUMMARY OF THE INVENTION

The present invention provides a method of setting the margins andplane-to-plane registration at factory calibration and in fieldadjustments. The present invention utilizes print element (PEL) sliceinsertion, mirror motor synchronization, and additional techniques toachieve the desired initial registration.

The invention comprises, in one form thereof, a method of setting aplurality of margins in an electrophotographic machine. A top margin fora reference color black is set by establishing a first time delaybetween a vertical synchronization signal and a first line. A rightmargin or a left margin for the reference color black is set byestablishing a second time delay between a horizontal synchronizationsignal and a start of printing. The other of the right margin and theleft margin is set by adjusting a scan speed of a laser beam across aphotoconductive element and adjusting a process speed in the cross-scandirection including speed of the photoconductive element, imageaccumulation member, and print medium. A bottom margin for the referencecolor black is set by adjusting the process speed. The other threecolors, cyan, magenta, and yellow, are then registered to the referencecolor black.

An advantage of the present invention is that margins and plane-to-planeregistration can be set at factory calibration and in field adjustmentsin order to compensate for manufacturing tolerances.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic, side view of one embodiment of a laser printer inwhich the method of the present invention may be used;

FIG. 2 is an overhead view of a sheet of print medium which may be usedin conjunction with the method of the present invention;

FIG. 3 is a schematic, side view of one embodiment of a factorycalibration fixture which may be used in conjunction with the method ofthe present invention;

FIG. 4 is a plot of a system clock signal and a reference clock signalwhich are used in conjunction with the method of the present invention;

FIG. 5 is an overhead view of a calibration page with which the top,left, right and bottom margins can be adjusted;

FIG. 6 is a lookup table for motor offsets;

FIG. 7 is a an overhead view of another calibration page with which thecolor planes can be adjusted to the black plane; and

FIG. 8 is a timing diagram of vertical synchronization signals which areused in conjunction with the method of the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and, more particularly, to FIG. 1, thereis shown one embodiment of a multicolor laser printer 10 in which themethod of the present invention may be used. Printer 10 includes laserprint heads 12, 14, 16, 18, a black toner cartridge 20, a magenta tonercartridge 22, a cyan toner cartridge 24, a yellow toner cartridge 26,photoconductive drums 28, 30, 32, 34, an intermediate transfer memberbelt 36 and a controller 37. The controller is a combination ofApplication Specific Integrated Circuits (ASIC's), microprocessors, andfirmware suited to the tasks described.

Each of laser print heads 12, 14, 16 and 18 projects a respective laserbeam 38, 40, 42, 44 off of a respective one of polygon mirrors 46, 48,50 and 52. As each of polygon mirrors 46, 48, 50 and 52 rotates, itscans a respective one of reflected laser beams 38, 40, 42 and 44 in ascan direction, perpendicular to the plane of FIG. 1, across arespective one of photoconductive drums 28, 30, 32 and 34. Each ofphotoconductive drums 28, 30, 32 and 34 is negatively charged toapproximately −1000 volts and is subsequently discharged to a level ofapproximately −300 volts in the areas of its peripheral surface that areimpinged by a respective one of laser beams 38, 40, 42 and 44. Duringeach scan of a laser beam across a photoconductive drum, each ofphotoconductive drums 28, 30, 32 and 34 is continuously rotated,clockwise in the embodiment shown, in a process direction indicated bydirection arrow 54. The scanning of laser beams 38, 40, 42 and 44 acrossthe peripheral surfaces of the photoconductive drums is cyclicallyrepeated, thereby discharging the areas of the peripheral surfaces onwhich the laser beams impinge.

The toner in each of toner cartridges 20, 22, 24 and 26 is negativelycharged and is transported upon the surface of a developer roll biasedto approximately −600 volts. Thus, when the toner from cartridges 20,22, 24 and 26 is brought into contact with a respective one ofphotoconductive drums 28, 30, 32 and 34, the toner is attracted to andadheres to the portions of the peripheral surfaces of the drums thathave been discharged to −300 volts by the laser beams. As belt 36rotates in the direction indicated by arrow 56, the toner from each ofdrums 28, 30, 32 and 34 is transferred to the outside surface of belt36. As a print medium, such as paper, travels along path 58, the toneris transferred to the surface of the print medium in nip 62.

In one embodiment of the method of the present invention, printerregistration adjustments are made at the end of the manufacturing lineto register the black plane to the paper, and set the plane-to-planeregistration of the cyan, magenta, and yellow planes (CMY) to the black(K) plane. The alignment process can be performed with the engine in 600dots per inch (dpi) mode. Targets are placed on the top, bottom, lefthand side and right hand side of the page (FIG. 2) which can used by themanufacturing line person when performing the calibration of themachine. The individual adjustments that are made are described below.

Printheads 12, 14, 16 and 18 are mechanically aligned and adjusted forskew using a calibration fixture prior to making any margin adjustments.Also, the initial setting for the right margin can be downloaded toprinter 10 during the same process used to align the printheads 12, 14,16, 18 to eliminate skew.

The skew for all four printheads is adjusted first mechanically using acalibration system that includes an alignment fixture with CCD camerasand LVDT sensors connected through a computer system. Second, furtherfine skew adjustment for the magenta, cyan and yellow printheads isaccomplished digitally using black as the reference. The fine skewadjustment has a range of +/−0.2117 mm in +/−10 increments of oneprinting element (PEL) each, with each PEL having a resolution of 1200dots per inch (dpi). After completing the coarse mechanical skewadjustment, the calibration system then measures the residual skew,calculates the desired adjustments for magenta, cyan, and yellow planes,and downloads the values to the machine where they are stored in NVRAM.

The printable area is aligned on the page, as illustrated in FIG. 2using the black plane as the reference.

The embodiment of the present invention described herein includes firstsetting the margins for the black plane and then aligning the colorplanes to black. However, it is to be understood that the sequence isnot limited to this order. Additionally, where operations are describedas being performed manually by an operator, it is to be understood thatthe same operations can be performed in an automated fashion using ascanner and settings communicated electronically to the printercontroller. In the illustrated embodiment, the image is scanned from theright margin to the left margin.

The margins can be aligned in the following order. First, in order toalign the black (K) top margin, the raster image processor (RIP)controller in controller 37 sets the delay from the verticalsynchronization signal (Vsync) to the first line that is printed on thepage. Second, to align the K right margin, the raster image processor(RIP) controller sets the time delay from the K horizontalsynchronization signal (Hsync). Third, to align the K left margin, theraster image processor (RIP).controller adjusts the speed of a motordriving polygon mirror 46, and an engine controller within controller 37adjusts the process speed of the page in process direction 54. Fourth,the engine controller also adjusts the process speed to align the Kbottom margin.

The black top margin is adjusted by changing the number of scan linesbetween the Vertical synchronization signal (Vsync) and the firstwriting line. The printed page consists of a logical page, physicalpage, and printable area. FIG. 2 illustrates the differences. Thelogical page describes the timing sequence for printing a page, thephysical page is the outline of the print medium to which the actualimage is transferred, and the printable area is defined as the printablearea on the page.

The logical page has a total of 6825 scan lines for a 279.4 mm (11 inch)page. This logical page is separated into a beginning buffer with apossible range of 125 scan lines (from the Vsync signal going low to thetop of the physical page), the 6600 scan lines for the physical page,and an ending buffer with a possible range of 100 scan lines at the endof the physical page. The top margin is adjusted by changing the size ofthe beginning and ending buffer (keeping the total of these two buffersequal to 6825−6600=225 scan lines) so that the physical page startsearlier or later in time relative to Vsync. This causes the printablearea to move towards the top or bottom of the page respectively. Theblack top margin adjustment has a range of +/−100 scan lines in +/−25increments with each increment corresponding to four 600 dpi scans. Fourscans at 600 dpi represents 0.0067 inch or 0.1693 mm in the processdirection 54.

The black top margin is set by first running a registration page (FIG.5). The registration page has alignment marks that indicate the top,right, left, and bottom margins. In this mode, the printable area isequal to the physical page and the operator will adjust the settingsuntil the alignment marks are at the edge of the page. If the operatordetermines that the top of the image is too far down the page, then thetop margin value will be decreased to pull the top of the image back tothe top of the physical page. Increasing the values will push the imagetoward the bottom of the page.

The panel range for the right margin is +/−12 increments. Each incrementcorresponds to four pels at a 600 dpi scan rate. Four pels at 600 dpirepresents 0.0067 inch or 0.1693 mm in scan direction 64. Each pel isdivided into a number of side-by-side slices (12 in this embodiment)extending in scan direction 64. The raster image processor (RIP)controls the right margin by adjusting the delay from the horizontalsynchronization signal, Hsync, to the first PEL. The right margin delay(RMD), expressed in slices, is calculated by the following Equation (1):RMD=RMO*4 pels/increment*12 slices/pel+RBL,wherein RMO is a right margin offset expressed in increments, and RBL isa right baseline number of slices determined to be the nominal delay.The factory calibration station transfers the RMO value to the rasterimage processor (RIP) at setup time.

At the factory calibration station, a factory calibration fixture 68(FIG. 3) is installed in printer 10 at the same location as thephotoconductor drum. More particularly, calibration gage 68 is insertedinto printer frame 70 in the place of photoconductive drum 28.

Laser printhead 12 produces a scanning laser beam 38 that reflects offof a folding mirror 72 onto an Hsync sensor 74. As printhead 12 scanslaser beam 38 in scan direction 64, two sensors 76, 78 in the form ofcharge coupled device (CCD) cameras sense a spot of laser beam 38 aslaser beam 38 travels across the active areas of cameras 76, 78. Thecalibration system then calculates the center of the laser spot todetermine where the spot is located as referenced to fixture 68 relativeto frame 70. The printer controller starts by turning on from PEL onethrough PEL 5100 with some nominal value loaded into the right margindelay (RMD) register (the printer scans from right margin to leftmargin). The printer controller 37 communicates to the calibrationsystem for feedback as to the location of the first PEL on first camera76. The controller 37 increments or decrements the right margin offset(RMO) value until the spot is at some predetermined location in theviewing area of first camera 76. The controller 37 then stores this asthe right RMO value in NVRAM.

After determining the black right margin offset (RMO), the calibrationsystem then looks for the end-of-scan PEL on the second camera 78 to setthe left margin using the black plane as the reference. The left marginadjustment range is +/−48 PELs in +/−12 increments with each incrementcorresponding to four 600 dpi PELs. The black left margin is adjusted bychanging the rotation speed of the printhead polygonal mirror motor withthe RIP controller and using the engine to compensate the process speedproportionally. The nominal mirror motor speed is 19500 RPM. At thenominal mirror motor speed the time for the laser beam to complete onescan (facet-to-facet) is 384.6 microseconds. This yields a nominal laserbeam velocity of 0.833 mm/microsecond. The time from the first to theend-of-scan PEL is constant for a given operating point and iscalculated to be 215.9 mm/0.833 mm/microsecond or 259.2 microseconds.For the nominal scan time of 384.6 microseconds the process speed needsto be set so that 6600 scan lines can be written on an 279.4 mm (11inch) page at 600 dpi. Therefore the nominal process speed is calculatedusing the following equation:Page Length/(Number of scan lines per page*scan time)279.4/(6600*0.0003846)=110.067 mm/second.

If the calibration system detennines that the black left margin needs tobe smaller which will increase the line length, then the calibrationsystem calculates the change in the left margin offset (LMO) to obtainthe correct line length. The calibration system will then instruct theRIP controller to increment the left margin offset (LMO) by that value.The RIP controller will then increase the mirror motor speedaccordingly. The increased mirror motor speed causes the scan time todecrease, which in turn increases the line length. The line lengthincreases because the laser beam's velocity increases. The calibrationsystem will then check the line length to determine if furtheradjustments are needed. This process is iterated until the last PEL iswithin tolerance for the left margin and the calibration systemcommunicates to the RIP controller to save the LMO in NVRAM.

Since the scan time has changed, the process speed needs to be adjustedso that the image will travel 279.4 mm in the time required to scan 6600scan lines. The RIP controller will communicate to the engine controllerthe value for the LMO so that the engine controller can compensate theprocess speed accordingly.

A reference clock for controlling the speed of the mirror motors and theprocess motors is generated by an ASIC in controller 37 using a counterand a system clock. A reference clock signal is generated by counting apredetermined number of system clock cycles and then toggling the outputstate of the reference clock after having counted the predeterminednumber of system clock cycles.

A mirror motor count (MMCOUNT) is defined as the number of cycles of thesystem clock that are to be counted before toggling the output of thereference clock that determines the speed of the mirror motor. A newmirror motor count (MMCOUNT) is calculated using a mirror motor count(BLMMCOUNT) at a baseline (nominal) line length and a baseline (nominal)operating speed, as well as a left margin offset (LMO) expressed inincrements. In the preferred embodiment, it has been determined thatincrementing the mirror motor count by four causes the line length tochange by approximately four pels. Then, by using a lookup table, theprint engine can change the process speed such that the number of scansbetween stations remains constant. The new mirror motor count (MMCOUNT)is given by the following Equation (2):MMCOUNT=BLMMCOUNT+(LMO*4 counts/increment).

A brushless direct current motor drives intermediate transfer belt 36and determines the speed of belt 36. A second reference clock generatesa signal which determines the speed of the motor that drives belt 36.The cycling of the second reference clock is governed by a belt motorcount (BMCOUNT) defined as the number of cycles of the system clock thatare to be counted before toggling the output of the second referenceclock. The belt motor count (BMCOUNT) is given by the following Equation(3):BMCOUNT=BLBMCOUNT+LMOBCOUNT,wherein BLBMCOUNT represents a baseline belt motor count between thetoggling of the state of the reference clock, and LMOBCOUNT represents aleft margin offset belt motor count corresponding to the left marginoffset LMO. The left margin offset belt motor count (LMOBCOUNT) isobtained from a lookup table which provides an offset value for everypossible value of left margin offset LMO, i.e., +/−12.

Similarly, a second brushless direct current motor drivesphotoconductive drum 28 and determines the speed of photoconductive drum28. A third reference clock generates a signal which determines thespeed of the motor that drives photoconductive drum 28. The cycling ofthe third reference clock is governed by a drum motor count (DMCOUNT)defined as the number of cycles of the system clock that are to becounted before toggling the output of the third reference clock. Thedrum motor count (DMCOUNT) is given by the following Equation (4):DMCOUNT=BLDMCOUNT+LMODCOUNT,wherein BLBDCOUNT represents a baseline drum motor count between thetoggling of the state of its reference clock, and LMODCOUNT represents aleft margin offset drum count corresponding to the left margin offsetLMO. The count (LMODCOUNT) is obtained from a lookup table whichprovides a LMODCOUNT value for every possible value of left marginoffset LMO, i.e., +/−12.

An example of the above-described calculations using actual numbers isnow provided in order to facilitate understanding. Assume that thesystem clock operates at a frequency of 16 MHz and that a referenceclock for driving the drum motor is to have a baseline frequency of839.735 Hz. The baseline drum motor count (BLDMCOUNT) for one half thereference clock period can then be calculated as follows:BLDMCOUNT=16 MHz/(839.735 Hz*2)=9527 counts.Thus, as shown in FIG. 4, the state of the drum motor reference clockchanges after every 9527 complete cycles of the system clock.

Further assume the following additional nominal settings at a resolutionof 600 dpi:Right baseline value (RBL)=2835 slices;Baseline mirror motor count (BLMMCOUNT)=5045 counts; andBaseline belt motor count (BLBMCOUNT)=10179 counts.Then, if the right margin offset (RMO) equals eight increments, and theleft margin offset (LMO) equals ten increments, then the followingvalues can be calculated:Right margin delay (RMD)=8 increments*4 pels/increment*12slices/pel+2835=3219 slicesMirror motor count (MMCOUNT)=5045+(4*10)=5085 countsBelt motor count (BMCOUNT)=10179+116=10295 countsDrum motor count (DMCOUNT)=9527+126=9653 countsThe values 116 and 126 from the previous two equations represent theleft margin offset belt count (LMOBCOUNT) and the left margin offsetdrum count (LMODCOUNT), respectively, and are obtained from the lookuptable (FIG. 6) which relates the left margin offset belt count(LMOBCOUNT) and the left margin offset drum count (LMODCOUNT) to theleft margin offset (LMO).

The panel range for the bottom margin is +/−25 increments. The fullrange of adjustment causes approximately a +/−2.5% change in the speedof the paper along the path. Therefore, with a page length of 279.4 mm,each increment causes a 0.1% change in paper speed and an approximate0.2794 mm shift in the bottom margin. This shift is accomplished bychanging the process speed for the belt motor, the drum motor and thepaper feed motor. Here, the paper feed motor speed is adjusted to feedthe print media in accordance with a speed change of the imageaccumulator belt. The count of the reference clock is increased by 0.1%per increment for the drum motor and for the belt motor, as indicated bythe following Equations (5) and (6):BMCOUNT=BLBMCOUNT+LMOBCOUNT+BMO*BLBMCOUNT*0.001DMCOUNT=BLDMCOUNT+LMODCOUNT+BMO*BLBMCOUNT*0.001,wherein BMO is a bottom margin offset expressed in increments. BMO isobtained from the registration page shown in FIG. 5. It is to beunderstood that it is Equations (5) and (6) that are actually used tocalculate BMCOUNT and DMCOUNT because they include both the left andbottom adjustments. Equations (3) and (4) cannot be used for setting thebottom margin.

It has been determined that by using the above incremental values, theprint engine can keep track of the number of scan lines between stationswith a simple equation using only integer math, as shown in Equation (7)below. In order to maintain the plane-to-plane registration, it isimperative that the print engine know the number of scan lines betweenthe color stations.SL=BLSL+2*LMO,wherein SL is the spacing in scan lines between color stations, and BLSLis the baseline or nominal spacing in scan lines between color stations.

Correction of the plane-to plane registration includes the process ofadjusting the three colors, CMY, to black. This can be performedpartially with the calibration system and then completed by printingcalibration patterns and measuring the difference between the planes. Asdescribed earlier, the black plane is registered to the paper and thenCMY is registered to the black plane. The calibration system can set theright and left margins but the top and bottom margins are adjusted usingthe calibration page shown in FIG. 5.

To adjust the CMY to the black plane a different calibration page shownin FIG. 7 is used. This page uses targets to align the top writing line,right margin (first PEL), and the left margin (line length).

The top margin has a coarse adjustment range of +/−127 increments of one600 dpi scan line per increment and a fine adjustment of +8 incrementsof ⅛ 600 dpi scan line per increment. The top writing line is adjustedby changing the delay between when the images are started relative tothe first plane that is imaged in the tandem process. For reference, theplanes are imaged in the following order: yellow, cyan, magenta, andblack. The nominal distance between the imaging stations is 101 mm or2385.8 600 dpi scan lines. Initially the planes are imaged at a nominaldelay of 2385 scan lines as shown in FIG. 8. While the calibrationsystem is adjusting the skew of the printheads, the distance between theimaging stations is measured. The difference between the measureddistance between the stations and the nominal distance is determined andthat value is converted to the number of 600 dpi scan lines. The integerportion is downloaded to the RIP controller to be used as the coarse topmargin offset (CTMO) for that respective plane as referenced to black.The fractional portion is converted to the nearest number of ⅛ scan lineincrements and that value is then downloaded to the RIP controller asthe fine top margin offset (FTMO). The calibration system will adjustthe CTMO so that the FTMO will always be positive. This adjustmentscheme-de-couples the black and CMY adjustments so that the adjustmentscan be executed in any order.

If, for example, the calibration system measures the distance betweenthe black and yellow stations to be 303.5 mm, then the calibrationsystem will determine that the distance is equal to 7169.3 scan lines.The nominal distance of 303 mm requires 7157.5 scan lines. Since theprocess speed is constant, this increased distance would cause theyellow top line to be late when it arrives at the black transfer point.To get the yellow to arrive on time, the yellow image needs to startearlier by the difference between the number of scan lines for thenominal distance minus the number of scan lines for the measureddistance. Also, since the fine adjustment is always positive, if themeasured distance is greater than the nominal distance, a count of oneshould be added to the measured distance. Therefore the CTMO is7157.5−7170.3 or −12.8 scan lines. The CTMO will be adjusted by −12 andthe FTMO will be 0.8*8 or 6.

After measuring the top margin, the calibration system then positionsthe right margin offset (RMO) for the CMY planes. The procedure issimilar to the right margin adjustment for the black plane except thatthe CMY RMO is measured as an error relative to the black plane andtherefore is additive to the black plane.

The last adjustment made by the calibration system is the line length orleft margin offset (LMO). The CMY line lengths are increased ordecreased by inserting PEL slices or removing PEL slices respectively tomatch the CMY line lengths to the black line length. Since all fourmirror motors are operating at the same synchronous speed and the CMYline lengths are measured as an error relative to the black line length,the CMY LMO is additive to the black left margin setting.

There is no adjustment of the bottom margin for colors.

If the margin and plane-to-plane settings need to be modified in thefield, then the settings can be modified by putting the printer in amaintenance mode, printing the registration pattern, and following thesteps in the procedure that were used in the factory calibration. In thefield, however, the service person determines the amount of correctionneeded by interpreting the printed registration pattern and thenentering the correction value by using the front operator panel. Thissame manual procedure may also be used in the printer manufacturingfacility when the calibration tool is not available.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A method of determining a length of a scan path of a laser beam in anelectrophotographic machine, said method comprising the steps of:placing a calibration fixture into a predetermined place in the machine,said predetermined place being designated for a photoconductive element;providing said calibration fixture with a first sensor and a secondsensor; detecting a start of the scan path with said first sensor;detecting an end of the scan path with said second sensor; andcalculating a length of the scan path dependent upon said detectingsteps, wherein said calculating step includes determining a number ofpel slices occurring between a first time of a horizontalsynchronization signal and a second time at which an end-of-scan pel ispositioned at a predetermined location on said second sensor.
 2. Amethod of determining a length of a scan path of a laser beam in anelectrophotogranhic machine, said method comprising the steps of:placing a calibration fixture into a predetermined place in the machine,said predetermined place being designated for a photoconductive element;providing said calibration fixture with a first sensor and a secondsensor; detecting a start of the scan path with said first sensor;detecting an end of the scan path with said second sensor; calculating alength of the scan path dependent upon said detecting steps; anddetermining a number of pel slices occurring between a first time of ahorizontal synchronization signal and a second time at which a pel ispositioned at a predetermined location on said first sensor.
 3. A methodof correcting registration in an electrophotographic machine having arotating mirror for scanning a laser beam along a length of a scan path,said method comprising the steps of: placing a calibration fixture intoa predetermined place in said machine, said predetermined place beingdesignated for a photoconductive element; providing said calibrationfixture with a first sensor spaced apart from a second sensor; detectinga start of said scan path with said first sensor; determine a rightmargin offset based on a location of said start; detecting an end ofsaid scan path with said second sensor; determining a left margin basedon a location of said end; adjusting said left margin by changing arotation speed of said rotating mirror to thereby adjust said length ofsaid scan path; and checking said length of said scan path.
 4. Themethod of claim 3, wherein said checking step comprises calculating saidlength of said scan path dependent upon said detecting steps.
 5. Themethod of claim 4, wherein said calculating step includes determining anumber of pel slices occurring between a first time of a horizontalsynchronization signal and a second time at which an end-of-scan pel ispositioned at a predetermined location on said second sensor.
 6. Themethod of claim 3, comprising the further step of calculating a centerof said scan path relative to said calibration fixture.
 7. The method ofclaim 3, comprising the further step of determining a number of pelslices occurring between a first time of a horizontal synchronizationsignal and a second time at which a pel is positioned at a predeterminedlocation on said first sensor.